Hollow waveguide and method of manufacturing the same

ABSTRACT

A hollow waveguide has a metal-clad pipe having an inside metal layer and an outside metal layer, and a hollow region formed inside of the metal-clad pipe. The metal-clad pipe is formed by pressure-bonding metal pipes each of which is made of a metal material different from each other.

The present application is based on Japanese patent application No.2004-152054, the entire contents of which are incorporated herein byreference.

The present Application is a Divisional Application of U.S. patentapplication Ser. No. 11/133,433, filed on May 20, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light transmitting medium in aninfrared wavelength band and, in particular, to a hollow waveguidesuitable for optical energy transmission of high power in the infraredwavelength band and a method of manufacturing the hollow waveguide.

Also, the present invention relates to a light transmitting medium in anultraviolet wavelength band and, in particular, to a hollow waveguidesuitable for optical energy transmission of high power in theultraviolet wavelength band and a method of manufacturing the hollowwaveguide.

2. Description of the Related Art

(1) Optical Waveguide for Transmitting Infrared Light

Infrared light having a wavelength of 2 μm or more is utilized invarious fields such as medicine, industrial working, measurement,analysis, and chemistry. In particular, each of an Er-YAG laser having awavelength band of 2.94 μm, a CO laser having a wavelength band of 5 μm,and a CO₂ laser having a wavelength band of 10.6 μm has high excitationefficiency and can develop high power and has large absorption also forwater, and hence is utilized as a light source for medical laserequipment and for industrial processing.

A conventional silica-based optical fiber used for communicationsproduces a large loss of transmission for laser light having awavelength of 2 μm or more because infrared absorption caused bymolecular vibration is large. For this reason, the usual silica-basedoptical fiber cannot be used as a waveguide for transmitting thisinfrared laser light. Hence, there have been actively made developmentsof a new type optical waveguide which can be applied even to an infraredwavelength band of wide application area.

A hollow waveguide provided with a dielectric layer transparent in thewavelength band of light to be transmitted has been developed as anoptical waveguide for transmitting infrared light having a wavelength of2 μm or more and has been proved to have excellent transmissioncharacteristics.

FIG. 5 is a sectional view showing a conventional hollow waveguide 4.The hollow waveguide 4 is constructed of a glass capillary 41, a metallayer 42 formed on the inside wall of the glass capillary 41, adielectric layer 43 formed on the metal layer 42, and a hollow region 44formed as a core inside the dielectric layer 43. The glass capillary 41is a base material for holding the mechanical strength of the hollowwaveguide 4. The dielectric layer 43 is transparent in the wavelengthband of light to be propagated and usually has a thickness of submicronor less and has a thickness set at an optimum value according to thewavelength of light to be propagated. The metal layer 42 has largeoptical absorption in the wavelength band of light propagating throughthe hollow waveguide 4, and hence optical energy does not penetratedeeply into the metal layer 42. Therefore, it is essential only that thethickness of the metal layer 42 in contact with the dielectric layer 43is a skin depth or more. Light propagating through the hollow waveguide4 is repeatedly reflected by the boundary between the hollow region 44and the dielectric layer 43 and the boundary between the dielectriclayer 43 and the metal layer 42, thereby being propagated.

Specifically, there is disclosed a hollow waveguide having the metallayer 42 made of silver on the inside wall of the glass capillary 41 byplating and the dielectric layer 43 formed on the metal layer 42 bythermosetting a solution of the precursor of polyimide or a solutionhaving an olefin polymer dissolved therein (for example, Japanese PatentApplication Laid-Open Nos. 8-234026 and 2002-71973).

Even if the metal layer 42 is formed on the glass capillary 41 having anextremely smooth surface by plating, the surface roughness of the metallayer 42 becomes larger as the thickness of the metal layer 42 becomeslarger. The metal layer 42 having a thickness of several hundredangstroms functions optically to a sufficient degree. Hence, the metallayer 42 is formed in as thin a thickness as possible so as to preventits surface from losing a mirror-smooth state.

An organic material constructing the dielectric layer 43 such aspolyimide and olefin polymer has an infrared absorption peak wavelengthspecific to the material. However, because the dielectric layer 43 isformed in a sufficiently thin thickness, propagating light in aninfrared range except for this infrared absorption peak wavelength ishardly attenuated in the dielectric layer 43. For this reason, thedielectric layer 43 can be assumed to be a transparent material throughwhich the propagating light reaches the metal layer 42. In particular,specific organic materials such as polyimide and olefin polymer do nothave a large infrared absorption peak in the oscillation wavelength bandof the Er-YAG laser, the CO laser, and the CO₂ laser, so that the hollowwaveguide 4 can transmit practically important infrared laser light withlow transmission loss. Moreover, since the metal layer 42 is formed in avery thin thickness by plating to keep the inside wall of the hollowwaveguide smooth, the hollow waveguide 4 can transmit not only infraredlaser light but also visible light as guide light.

In addition to the hollow waveguide 4 having the dielectric layer 43made of the organic material as described above, a hollow waveguidehaving a dielectric layer formed by chemically altering a part of ametal layer is also developed. For example, there has been known ahollow waveguide in which a silver iodide layer produced by iodinating apart of a metal layer, which is made of silver on the inside wall of aglass capillary by plating, functions as a transparent dielectric layer.The silver iodide is an inorganic substance which is transparent in theinfrared wavelength band and does not have an infrared absorption peakspecific to the substance like a polymer substance. Thus, the silveriodide can transmit light in the infrared wavelength range with lowtransmission loss.

In addition to the above-described glass capillary, a resin tube made offluorine resin or the like having excellent flexibility is proposed as abase material for providing the hollow waveguide with the mechanicalstrength. Further, for a laser probe mounted at the tip of a longoptical transmission line and uses not specially requiring flexibility,there is proposed a base material made by polishing the inside surfaceof a stainless pipe or the like which is mechanically stronger than theglass capillary. Still further, there is proposed a hollow waveguide inwhich a pipe itself made of precious metal such as silver is used as abase material so as to save a step of forming a silver layer by platingand has its inside surface polished to a mirror-smooth state to form adielectric layer.

(2) Optical Waveguide for Transmitting Ultraviolet Light

On the other hand, ultraviolet light having a wavelength of 250 nm orless is utilized in various fields such as medicine, industrialprocessing, measurement, analysis, and chemistry. In particular, a KrFlaser having a wavelength of 248 nm, an ArF laser having a wavelengthband of 193 nm, an excimer laser such as an F2 laser having a wavelengthband of 157 nm, or a Q switch YAG harmonic laser can produce high powerand is important as a light source for semiconductor photolithographyequipment, fluorescence analysis, medical equipment, and industrialprocessing.

A conventional silica-based optical fiber used for communications cantransmit light having a wavelength of approximately 200 nm or more withlow transmission loss. Moreover, recently, a solid-type silica-basedoptical fiber has been improved particularly for the purpose oftransmitting ultraviolet light.

Absorption in an ultraviolet band is an absorption band caused byelectron transition and absorption spectrum characteristic issubstantially affected by impurities and structural defects contained inthe silica-based optical fiber. Silica-based optical fibers widely usedat present have been purified to such an extent that absorption bymetallic impurities can be neglected. Hence, transmittance in theultraviolet band of the silica-based optical fiber is determined bystructural defects in silica glass that depend on the manufacturingconditions.

Fine structural defects depend on the manufacturing conditions: forexample, oxygen depletion type defects and oxygen excess type defectsare caused by an oxidizing/reducing atmosphere when the fiber ismanufactured. In a method of manufacturing an optical fiber in whichsoot-like silica particles are dehydrated in a halogen atmosphere andthen are heat-treated to be altered to transparent glass, oxygendepletion type defects are caused to decrease transmittance in theultraviolet band. Because the content of an OH group is varied by thedehydration treatment, the transmittance in the ultraviolet band dependson the OH group.

In silica glass anhydride subjected to the dehydration treatment, anabsorption band caused by Si—Si oxygen depletion type defects isobserved at wavelengths of 245 nm and 163 nm. Further, in silica glassmade by sintering soot in a reducing atmosphere, an absorption band isobserved at a wavelength of 240 nm.

In contrast to this, silica glass made by sintering soot in a He gasatmosphere contains a high concentration of the OH group and does nothave an outstanding absorption band observed in a wavelength range from200 nm to 400 nm. As described above, the transmittance of thesilica-based optical fiber for the purpose of transmitting ultravioletlight depends on the content of the OH group.

On the other hand, apart from the solid-type silica-based optical fiberlike this, there is proposed, for the purpose of transmittingultraviolet light, a hollow waveguide made by depositing aluminum on theinside of a hollow glass capillary by a metal organic chemical vapordeposition method (MOCVD) (Optical Alliance, July 1999, pp. 20-22). Theadvantage of the hollow waveguide can endure higher energy density thanthe silica-based optical fiber. The maximum transmission energy densityof the solid-type silica-based optical fiber is approximately 50 mJ/cm²,whereas the hollow waveguide can transmit a beam having a transmissionenergy density of 2 J/cm² or more. Further, even in the case of thesilica-based optical fiber improved for the ultraviolet light, thewavelength of 193 nm of the ArF laser is the shortest limit and it isdifficult to transmit light in the vacuum ultraviolet band of shorterwavelength. In contrast to this, the hollow waveguide having an aluminumthin film deposited thereon can transmit light having as short awavelength as approximately 130 nm and can transmit also the F2 laserhaving a wavelength of 157 nm.

(1) Problems of Hollow Waveguide for Transmitting Infrared Light

However, the conventional hollow waveguide for transmitting infraredlight has the following problems. That is, the hollow waveguide 4 usingthe glass capillary 41 has flexibility but has a possibility of beingsuddenly broken when it is held in a small bending radius for a longtime. Further, the hollow waveguide 4 has a possibility of being brokenwhen it is inserted into the human body or used in use environment whereimpact or external force is applied thereto, which is not so desirable.

A hollow waveguide using a resin tube made of fluorine resin or the likeas a base material has a lower possibility of being broken than a hollowwaveguide using a glass capillary but is irregularly varied in asectional shape and in the bending shape of the whole transmission lineby impact or external force, thereby being easily varied in transmissioncharacteristics. Further, in the resin tube, as compared with the glasscapillary, the surface of the inside wall is rough and is hard toimprove by polishing or etching to such an extent that is achieved inthe glass capillary. For this reason, transmission loss is increased inthe case of transmitting light having short wavelengths such as visiblelight.

Moreover, the entire loss of light propagating in the hollow waveguideis converted to heat, and hence the hollow waveguide using a glasscapillary or a resin tube having small thermal conductivity as a basematerial might cause local heating.

In the hollow waveguide formed of the glass capillary, the silver layerformed on the inside wall of the glass capillary, and the dielectriclayer made of silver iodide formed by iodinating the inside wall of thesilver layer, the silver layer needs to be formed in a thickness largerthan an optically contributable thickness so as to avoid the silverlayer to be iodinated from being lost. As a result, the surface of theinside wall of the hollow waveguide having silver iodide applied theretois degraded in the roughness to damage the mirror-smooth state of thesilver layer, which is disadvantageous specially for the transmission oflight having short wavelengths such as visible light.

When a hollow waveguide is used for use not requiring flexibility, ametal pipe can be advantageously used as the base material of the hollowwaveguide in that the metal pipe has large mechanical strength and highthermal conductivity. However, a conventional hollow waveguide, whichuses a stainless pipe having its inside surface polished to amirror-smooth state as a base material and has a silver layer formed onthe inside wall of the stainless pipe by plating, loses the smoothnessof the surface of the inside wall by plating and hence is remarkablyinferior in smoothness to the hollow waveguide formed by plating theglass capillary with silver.

FIG. 6 shows a wavelength-loss characteristic when white light ispropagated through a metal hollow waveguide, which is formed of astainless pipe having its inside wall polished to a mirror-smooth stateand a silver layer formed by plating the inside wall of the stainlesspipe with silver. FIG. 7 shows a wavelength-loss characteristic whenwhite light is propagated through a metal hollow waveguide, which isformed of a glass capillary and a silver layer formed by plating theinside wall of the glass capillary with silver. Each of the metal hollowwaveguides has a length of 40 cm and an inside diameter of 0.7 mm andthe same thickness of the silver layer formed by plating.

As shown in FIGS. 6 and 7, the loss of the metal hollow waveguide formedof the stainless pipe is considerably larger than the loss of the metalhollow waveguide formed of the glass capillary. In particular, as thewavelength becomes shorter, the loss becomes larger. It is thought thatthis is because the inside surface of the stainless pipe is not polishedto the same degree of smoothness equal as in the case of the glasscapillary or that this is because even if the stainless pipe and theglass capillary are equal to each other in the degree of smoothness, thesurface roughness of the silver layer is different between the stainlesspipe and the glass capillary because of difference in the base material,that is, the smoothness of the base material cannot be held on thesurface of the silver layer. With this characteristic, the hollowwaveguide formed of the stainless pipe having its inside surfacepolished to a mirror-smooth state and the silver layer formed by platingthe polished inside surface of the pipe with silver is inferior intransmission loss to the hollow waveguide formed of the glass capillaryand the silver layer formed by plating the glass capillary with silver.

Further, the hollow waveguide formed of the glass capillary has problemsof being lower in resistance to external force, easily causing localheating because of using glass having low thermal conductivity as thebase material, and silver plating being easily peeled off.

Still further, a hollow waveguide is also studied in which in place ofthe hollow waveguide including a stainless pipe having its insidesurface polished and a silver layer formed by plating the polishedinside surface with silver, a silver pipe itself has its inside surfacepolished to eliminate a step of plating the inside surface with metal tokeep the surface roughness of the inside wall of the pipe. In thishollow waveguide, the whole base material is silver and hence cost isvery much increased. It is known that not only silver but also gold andcopper are suitable as such a metal material of a hollow waveguide thatis optically contributable to transmitting a laser light in an infraredwavelength band with low loss. It is difficult in practical use in termsof cost to form the base material of the hollow waveguide of gold.Further, silver and copper are remarkably discolored by oxidation orsulfuration, and therefore, it is not preferable that the base materialof the hollow waveguide is made of these materials and exposed tooutside environment. Still further, the hollow waveguide using any oneof these materials as the base material is easily plastically deformedeven by small bending and hence is remarkably degraded in transmissioncharacteristics particularly in use environment where the hollowwaveguide is repeatedly bent.

(2) Problems of Hollow Waveguide for Transmitting Ultraviolet Light

A conventional hollow waveguide for transmitting ultraviolet light hasthe following problems.

That is, when a pulse of ultraviolet light is entered into a silicaoptical fiber commonly used, even if an initial transmittance isexcellent, the transmission characteristic is degraded as time elapsesduring the irradiation of light (Appl. Opt. 27, 1988, p. 3124).

As described above, there have been developed also silica optical fibersfor transmitting ultraviolet light that can transmit even ultravioletlight with stability by adjusting the concentration of the OH group.However, such silica optical fibers yet have problems in transmittingthe ArF laser and the KrF laser having wide application fields. Further,the silica optical fibers can not stably transmit the F2 laser having ashort wavelength and a high-power pulse laser for a long time.

On the other hand, an aluminum hollow waveguide is more promising in thetransmission of ultraviolet laser of 190 nm or less in wavelength or ofhigh power intensity than the silica optical fiber. However, in theabove-described hollow waveguide having an aluminum thin film depositedinside the silica glass capillary by the MOCVD method, the aluminum thinfilm does not have a sufficiently strong adhesion force and hence iseasily peeled off. In particular, in the case of transmitting theultraviolet light by the hollow waveguide, the hollow space is commonlyevacuated to a vacuum or filled with rare gas so as to prevent oxygen inthe air from being altered to ozone absorbing the ultraviolet light toincrease transmission loss. For this reason, there is a possibility thatwhen the aluminum thin film deposited inside does not have asufficiently strong adhesion force, the aluminum thin film might bepeeled off when the gas is sucked or introduced.

Further, to deposit the aluminum thin film inside the silica glasscapillary, an expensive MOCVD apparatus is required.

Still further, the hollow waveguide using the glass capillary like thisis low in resistance to the external force and hence might be broken byimpact or bending.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a hollow waveguide that ismechanically strong, is not broken even in the bending of a smallbending radius, is excellent in thermal conductivity, and is capable oftransmitting propagating light with low transmission loss, and a methodof manufacturing the hollow waveguide.

It is another object of the invention to provide a hollow waveguide thatcan keep stable transmission efficiency also for ultraviolet laser lighthaving high peak power and a short wavelength for a long time and amethod of manufacturing the hollow waveguide.

(1) According to One Aspect of the Invention, a Hollow WaveguideComprises:

a metal-clad pipe comprising an inside metal layer and an outside metallayer; and

a hollow region formed inside of the metal-clad pipe,

wherein the metal-clad pipe is formed by pressure-bonding metal pipeseach of which comprises a metal material different from each other.

Herein, the pressure-bonding means a process that can be performed byany of known methods of bonding two metal pipes with different diametersby applying pressure and thereby forming them into a single metal pipewith an inside metal layer and an outside metal layer, i.e., themetal-clad pipe. For example, the known methods can include extrudingand rolling.

(2) According to Another Aspect of the Invention, a Hollow WaveguideComprises:

a metal-clad pipe comprising an inside metal layer and an outside metallayer;

a dielectric layer formed on an inner wall of the metal-clad pipe; and

a hollow region formed inside of the dielectric layer,

wherein the metal-clad pipe is formed by pressure-bonding metal pipeseach of which comprises a metal material different from each other.

(3) According to Another Aspect of the Invention, a Method ofManufacturing a Hollow Waveguide Including a Metal Pipe and a HollowRegion Formed Inside of the Metal Pipe, Comprises the Steps of:

pressure-bonding metal pipes each of which comprises a metal materialdifferent from each other to form a metal-clad pipe comprising an insidemetal layer and an outside metal layer; and

polishing a surface of the inside metal layer.

(4) According to Another Aspect of the Invention, a Method ofManufacturing a Hollow Waveguide Including a Metal Pipe and a HollowRegion Formed Inside of the Metal Pipe, Comprises the Steps of:

pressure-bonding metal pipes each of which comprises a metal materialdifferent from each other to form a metal-clad pipe comprising an insidemetal layer and an outside metal layer;

polishing a surface of the inside metal layer; and

forming a dielectric layer on the polished surface of the inside metallayer.

(5) According to Another Aspect of the Invention, a Hollow WaveguideComprises:

a metal-clad pipe comprising an inside metal layer and an outside metallayer each of which comprises a metal material different from eachother; and

a hollow region formed inside of the metal-clad pipe,

wherein strength of bonding between the outside metal layer and theinside metal layer is 10 MPa or more.

It is preferable that the inside metal layer is made of a metal materialhaving a large absolute value of complex index of refraction.

The inside metal layer may comprise gold, silver, or copper, and theoutside metal layer may comprises stainless steel, phosphorous bronze,titanium, or titanium alloy.

The inside metal layer may comprise aluminum, and the outside metallayer may comprise stainless steel, phosphorous bronze, titanium, ortitanium alloy.

The inside metal layer may comprise silver, and the dielectric layer maycomprise silver iodide.

The inside metal layer may comprise aluminum, and the dielectric layermay comprise aluminum oxide.

The dielectric layer comprising aluminum oxide preferably comprises athickness of 0.1 μm or less.

According to the present invention, there is provided a hollow waveguidethat is mechanically strong, is not broken even in the bending of asmall bending radius, is excellent in thermal conductivity, and iscapable of transmitting propagating light with low transmission loss.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explainedbelow referring to the drawings, wherein:

FIG. 1 is a sectional view showing a hollow waveguide of a firstembodiment according to the present invention;

FIG. 2 is a wavelength-loss characteristic diagram of the silver hollowwaveguide using a metal-clad pipe comprising an inside silver layer andan outside stainless-steel layer as the base material according to thepresent invention;

FIGS. 3A to 3C are sectional views showing the steps of manufacturingthe hollow waveguide of the first embodiment;

FIG. 4 is a sectional view showing a hollow waveguide of a secondembodiment according to the present invention;

FIG. 5 is a sectional view showing the conventional hollow waveguide;

FIG. 6 is a wavelength-loss characteristic diagram of the silver hollowwaveguide using a stainless steel as the base material of the hollowwaveguide which is made by the conventional method; and

FIG. 7 is a wavelength-loss characteristic diagram of the silver hollowwaveguide using a glass capillary as the base material of the hollowwaveguide which is made by the conventional method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be described in detail withreference to the accompanying drawings.

FIRST EMBODIMENT

FIG. 1 is a sectional view showing a hollow waveguide 1 of a firstembodiment according to the present invention.

As shown in FIG. 1, the hollow waveguide 1 is formed of a silver-cladstainless pipe (metal-clad pipe) 10, which is integrally formed as ametal pipe made of a silver-clad layer 12 and a stainless layer 11 bypressure-bonding (e.g., extruding or rolling) a cylindrical silver pipearranged inside to a stainless pipe arranged outside, and an olefinpolymer layer formed as a dielectric layer 13 on the inside wall of themetal-clad pipe 10. A hollow region 14 inside the dielectric layer 13corresponds to a core for propagating light.

A method of manufacturing the hollow waveguide 1 will be described.

As shown in FIG. 3A, in this embodiment, first, two metal pipes of astainless pipe 16 and a silver pipe 15 having an outside diametersmaller than the inside diameter of the stainless pipe 16 are prepared,then the silver pipe 15 is inserted into the stainless pipe 16, and thenthe stainless pipe 16 is extruded to form a two-layer laminated pipe inwhich a stainless layer is pressure-bonded onto a silver layer.Thereafter, the two-layer laminated pipe is repeatedly drawn to adesired final shape to produce a silver-clad stainless pipe (metal-cladpipe) 10 having a small diameter.

A layer formed of the inside silver pipe 15 and a layer formed of theoutside stainless pipe 16 of the formed silver-clad stainless pipe 10are called a silver-clad layer 12 and a stainless layer 11,respectively. The strength of bonding between the silver-clad layer 12and the stainless layer 11 is 10 MPa or more. The thickness of thesilver-clad layer 12 in the silver-clad stainless pipe 10 issufficiently larger than a silver film formed by plating. Hence, even ifthe silver-clad stainless pipe 10 is subjected to bending or otherworking, the silver-clad layer 12 is not peeled off.

The hollow waveguide 1 is formed in the following sizes: for example,the outside diameter and the inside diameter of the metal-clad pipe 10are 1.1 mm and 0.66 mm, respectively; the thickness of the stainlesslayer 11 is 0.15 mm; and the thickness of the silver-clad layer 12 is0.07 mm. The thickness of the silver-clad layer 12 is preferably made0.05 mm or more to make allowance for polishing and is preferably madesmaller than the thickness of the stainless layer 11 so that thesilver-clad layer 12 and the stainless layer 11 are concentricallyuniformly formed and that the silver-clad layer 12 is prevented frombeing deformed and peeled off by bending or other working.

In general, gold, silver, and copper which are used as the materials ofthe inside metal pipe suitable for a hollow waveguide are soft and henceare easily plastically deformed as compared with stainless steel,phosphor bronze, titanium, and titanium alloy which are used as thematerials of the outside metal pipe. Hence, the thickness of the insidemetal pipe is preferably made ½ or less times smaller than the thicknessof the outside metal pipe.

Next, as shown in FIG. 3B, the inside wall of the silver-clad layer 12is mechanically chemically polished to a mirror-smooth state. Thisprocess uses elution by chemical polishing and abrasion action byabrasives in combination and can not only smooth the inside surface butalso prevent part of the silver-clad layer 12 from developing awork-altering layer. When comparing roughness on the inside surface ofthe silver-clad layer 12 before and after polishing, an arithmeticalmean deviation of profile Ra could be reduced from 1.1 μm to 0.001 μmand a maximum roughness Rmax could be reduced from 8.9 μm to 0.03 μm.The hollow waveguide according to the present invention mainly aims topropagate infrared light but at the same time can be applied also to thepropagation of visible light having a shorter wavelength as guide light.Hence, as to inside surface roughness, it is preferable that Ra ispreferably made 1/200 or less times smaller than the wavelength of lightto be propagated and that Rmax is made 1/20 or less times smaller thanthe wavelength of light to be propagated. Assuming that an allowance of0.02 mm is made for a decrease in the thickness of the polishedsilver-clad layer 12, the resultant final inside diameter of thesilver-clad stainless pipe 10 is made 0.7 mm.

Finally, as shown in FIG. 3C, a solution into which an olefin polymer isdissolved is poured inside the silver-clad stainless pipe 10 having itsinside surface polished to a mirror-smooth state and then is cured byheat treatment. With this, the hollow waveguide 1 having the surface ofthe silver-clad layer 12 coated with an olefin polymer layer (dielectriclayer 13) is produced. The film thickness of the dielectric layer 13 ismade 0.3 μm in consideration of the wavelength of light to be propagatedso as to transmit CO₂ laser light having a wavelength of 10.6 μm.

Next, the operation of this embodiment will be described.

The hollow waveguide 1 propagates laser light with the metal-clad pipe10 and the dielectric layer 13 used as a clad layer and with the hollowregion 14 used as a core. More specifically, the laser light isrepeatedly reflected by the boundary between the hollow region 14 andthe dielectric layer 13 and by the boundary between the dielectric layer13 and the silver-clad layer 12, thereby being propagated in thedirection of propagation (in the longitudinal direction of the hollowwaveguide 1). The hollow waveguide 1 of this embodiment can transmit CO₂laser light having a wavelength of 10.6 μm through a waveguide length of40 cm at a transmittance of 95% or more.

Because the stainless pipe 16 is used as the base material of the hollowwaveguide 1, the hollow waveguide 1 is mechanically strong and is notplastically deformed by the bending of a small bending radius, externalpressure, or the like and hence is hardly broken or degraded intransmission characteristics. Moreover, both of the stainless layer 11and the silver-clad layer 12 forming the metal-clad pipe 10 are made ofmetal having large thermal conductivity and hence can prevent localheating. Because the metal-clad pipe 10 has its inside metal layer madeof silver having a large absolute value of complex index of refraction,the metal-clad pipe 10 can be made a metal-clad pipe having a largerefractive index to reduce radiation loss in propagating light.Moreover, because the silver pipe 15 is pressure-bonded (e.g., extrudedor rolled) onto the stainless pipe 16 to form the metal-clad pipe 10,the silver pipe 15 is hardly peeled off from the stainless layer 11 thatis the base material of the silver-clad layer 12.

Here, the wavelength-loss characteristic of the silver hollow waveguide1 shown in FIG. 2 is compared with the wavelength-loss characteristicsshown in FIGS. 6 and 7.

FIG. 2 is the wavelength-loss characteristic when white light ispropagated through the silver hollow waveguide (metal-clad pipe) 10having the inside wall of the silver-clad layer 12 polished to amirror-smooth state. As shown in FIG. 2, the wavelength-losscharacteristic of the silver hollow waveguide 10 is a characteristic inwhich loss is increased in a short wavelength band but is smaller in anywavelength band as compared with the wavelength-loss characteristic of asilver hollow waveguide shown in FIG. 6 and having a silver layer formedon the inside wall of the stainless pipe by plating. Moreover, ascompared with the wavelength-loss characteristic of a silver hollowwaveguide shown in FIG. 7 and having a silver layer formed on the insidewall of a glass capillary by plating, propagation loss is onapproximately the same level between them. Hence, the hollow waveguide 1of this embodiment has an advantage of using the stainless pipe as thebase material and can transmit light with low transmission loss.

Further, from the characteristic curve shown in FIG. 2, it is found thatthe refractive index of the silver-clad layer 12 of this embodiment iscloser to the refractive index of silver in the form of bulk than therefractive index of the silver layer formed by plating.

Still further, according to the method of manufacturing the hollowwaveguide 1 of this embodiment, as described above, it is possible tomanufacture a hollow waveguide that is strong in the mechanicalstrength, is resistant to be deformed by the bending of a small bendingradius, impact, and external pressure, and also in the opticalcharacteristic, can propagate light from a visible light wavelength bandto an infrared wavelength band with low propagation loss.

While the olefin polymer is used for the dielectric layer 13 in thehollow waveguide 1 of this embodiment, it is also recommended as amodification that part of the sliver-clad layer is iodinated to form adielectric layer made of silver iodide on the inside wall of thesilver-clad layer.

A method of manufacturing the hollow waveguide having this dielectriclayer made of silver iodide is the same as the method of manufacturingthe hollow waveguide 1 until the step shown in FIGS. 3A and 3B, andthereafter, the inside wall of the silver pipe is polished and then partof the silver-clad layer is chemically altered into silver iodide. Inthis method, part of the silver-clad layer is chemically altered to forma silver iodide layer (dielectric layer), and hence the silver-cladlayer needs to be formed in consideration of the thickness of the silveriodide layer.

In a conventional hollow waveguide made by forming a silver-plated layeron the inside wall of a glass capillary and by chemically altering partof the silver-plated layer into a silver iodide layer, the thickness ofthe silver-plated layer needs to be made larger by the thickness ofsilver iodide layer, and hence the surface of the inside wall of thesilver-plated layer becomes rough. As compared with this conventionalhollow waveguide, in this embodiment, the silver-clad stainless pipehaving the inside wall polished to a mirror-smooth state is used andhence can keep the inside wall in the mirror-smooth state, whichresulting in making the inside wall smoother than a case where theinside wall of the glass capillary is plated with silver and hence inbeing able to transmit laser light with low loss. Moreover, there isproduced the same effect as the above-described hollow waveguide 1.

While the silver pipe 15 is used for the inside clad layer of themetal-clad pipe in the hollow waveguide 1 of this embodiment, even if apipe made of not only silver but also metal having a large absolutevalue of complex index of refraction such as gold and copper is used inthe hollow waveguide not using the dielectric layer made of silveriodide, the pipe can similarly transmit laser light in the infraredwavelength band with low loss.

Moreover, while the stainless pipe 16 is used for the outside metal pipein the hollow waveguide 1 of this embodiment, the material to be used isnot limited to the stainless pipe but it is also recommendable to use aphosphorous bronze pipe resistant to plastic deformation, titanium whichis nontoxic even if it is inserted into a human body, or a titaniumalloy such as nickel titanium.

SECOND EMBODIMENT

FIG. 4 is a sectional view showing a hollow waveguide 21 of a secondembodiment according to the present invention. In FIG. 4, the sameconstituent elements as shown in FIG. 1 are denoted by the samereference symbols as used in FIG. 1.

As shown in FIG. 4, the hollow waveguide 21 is formed of analuminum-clad stainless pipe (composite metal pipe) 20, which isintegrally formed as a metal pipe made of an aluminum-clad layer 22 anda stainless layer 11 by pressure-bonding (e.g., extruding or rolling) acylindrical aluminum pipe arranged inside to a stainless pipe arrangedoutside, and having the inside wall surface of the aluminum-clad layer22 polished. The inside wall surface of the aluminum-clad layer 22 isoxidized to form an aluminum oxide (Al₂O₃) layer 23 and ultravioletlight propagates through the hollow region 14.

Next, a method of manufacturing this hollow waveguide 21 will bedescribed.

In this embodiment, first, two metal pipes of a stainless pipe and analuminum pipe having an outside diameter smaller than the insidediameter of the stainless pipe are prepared, the aluminum pipe isinserted into the stainless pipe, and then the stainless pipe isextruded to form a two-layer laminated pipe in which a stainless layeris pressure-bonded (e.g., extruded or rolled) onto an aluminum layer.Thereafter, the two-layer laminated pipe is repeatedly drawn to adesired final shape to produce an aluminum-clad stainless pipe 20 havinga small diameter.

A layer formed of the inside aluminum pipe and a layer formed of theoutside stainless pipe of the formed aluminum-clad stainless pipe 20 arecalled an aluminum-clad layer 22 and a stainless layer 11, respectively.The strength of bonding between the aluminum-clad layer 22 and thestainless layer 11 is 10 MPa or more. The thickness of the aluminum-cladlayer 22 in the aluminum-clad stainless pipe 20 is sufficiently largerthan an aluminum film formed in a glass capillary by a MOCVD method.Hence, even if the aluminum-clad stainless pipe 20 is subjected tobending or other working, the aluminum-clad layer 22 is not peeled off.

The hollow waveguide 21 is formed in the following sizes: for example,the outside diameter and the inside diameter of the aluminum-cladstainless pipe 20 are 1.1 mm and 0.66 mm, respectively; the thickness ofthe stainless layer 11 is 0.15 mm; and the thickness of thealuminum-clad layer 22 is 0.07 mm. The thickness of the aluminum-cladlayer 22 is preferably made 0.05 mm or more to make allowance forpolishing.

Moreover, the thickness of the aluminum-clad layer 22 is preferably madesmaller than the thickness of the stainless layer 11 so that thealuminum-clad layer 22 and the stainless layer 11 are concentricallyuniformly formed and that the aluminum-clad layer 22 is prevented frombeing deformed and peeled off by bending or other working. In additionto the stainless steel, phosphorous bronze hard to deform, titaniumwhich is nontoxic and safe even if it is inserted into the human bodyand is light in weight, and titanium alloy such as nickel titanium areused as the materials of the outside metal pipe. Aluminum is softer andis more easily plastically deformed as compared with these materialsexpected to be used as the material of the outside metal pipe, and hencethe thickness of the inside metal pipe (aluminum pipe) is preferablymade ½ or less times smaller than the thickness of the outside metalpipe.

Next, the inside wall surface of the aluminum-clad layer 22 ismechanically chemically polished to a mirror-smooth state. This processuses elution by chemical polishing and abrasive action by abrasives incombination. When comparing roughness on the inside surface before andafter polishing, an arithmetical mean deviation of profile Ra could bereduced from 1.1 μm to 0.01 μm and a maximum roughness Rmax could bereduced from 9 μm to 0.03 μm. Assuming that an allowance of 0.02 mm ismade for a decrease in the thickness of the polished aluminum-clad layer22, the resultant final inside diameter of the aluminum-clad stainlesspipe 20 is made 0.7 mm.

More preferably, the aluminum-clad stainless pipe 20 having its insidewall surface finally polished to a mirror-smooth state is subjected tohigh-temperature heat treatment with steam flowed inside to oxidize theinside wall surface of the aluminum-clad stainless pipe 20 to form analuminum oxide layer 23. With this, it is possible to prevent thechemical alteration of the aluminum-clad layer 22 and to preventabrasion caused by ultraviolet laser light of high power. This effectcan be sufficiently produced by the aluminum oxide layer 23 having athickness of 0.1 μm or less.

Next, the effect of this embodiment will be described.

Because the stainless pipe is used as the base material of the hollowwaveguide 21, the hollow waveguide 21 is mechanically strong and isresistant to being deformed by the bending of a small bending radius,external pressure, or the like, and hence is hardly broken or degradedin transmission characteristics.

Further, both of the stainless layer 11 and the aluminum-clad layer 22are made of metal having large thermal conductivity and hence canprevent local heating even if they are heated by transmitting laserlight.

Still further, because the aluminum pipe is pressure-bonded (e.g.,extruded or rolled) onto the stainless pipe to form the aluminum-cladstainless pipe 20, the aluminum-clad layer 22 is resistant to beingpeeled off from the stainless layer 11 of the base material.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A method of manufacturing a hollow waveguide including a metal pipeand a hollow region formed inside of the metal pipe, comprising:pressure-bonding metal pipes each of which comprises a metal materialdifferent from each other to form a metal-clad pipe comprising an insidemetal layer and an outside metal layer; and polishing a surface of theinside metal layer.
 2. A method of manufacturing a hollow waveguideincluding a metal pipe and a hollow region formed inside of the metalpipe, comprising: pressure-bonding metal pipes each of which comprises ametal material different from each other to form a metal-clad pipecomprising an inside metal layer and an outside metal layer; polishing asurface of the inside metal layer; and forming a dielectric layer on thepolished surface of the inside metal layer.
 3. The method according toclaim 1, wherein: the inside metal layer comprises a metal materialhaving a large absolute value of complex index of refraction.
 4. Themethod according to claim 1, wherein: the inside metal layer comprisesgold, silver, or copper, and the outside metal layer comprises stainlesssteel, phosphorous bronze, titanium, or titanium alloy.
 5. The methodaccording to claim 1, wherein: the inside metal layer comprisesaluminum, and the outside metal layer comprises stainless steel,phosphorous bronze, titanium, or titanium alloy.
 6. The method accordingto claim 2, wherein: the inside metal layer comprises silver, and thedielectric layer comprises silver iodide.
 7. The method according toclaim 4, wherein: the inside metal layer comprises silver, and thedielectric layer comprises silver iodide.
 8. The method according toclaim 2, wherein: the inside metal layer comprises aluminum, and thedielectric layer comprises aluminum oxide.
 9. The method according toclaim 5, wherein: the inside metal layer comprises aluminum, and thedielectric layer comprises aluminum oxide.
 10. The method according toclaim 8, wherein: the dielectric layer comprises a thickness of 0.1 μmor less.